Complete `dup2` system call
Complete `dup2` system call

Add support for duplicating a file descriptor if the destination file
descriptor does not yet exist. Also adjust the spacing to 4 space tabs

TheDude05/xv6,williamsandrew/xv6,williamsandrew/xv6,TheDude05/xv6,TheDude05/xv6,williamsandrew/xv6